ECR plasma generating devices have been developed for use in semiconductor manufacturing processes such as chemical vapor deposition and etching.
U.S. Pat. No. 5,133,826 to Dandl describes an ECR plasma source having magnets disposed circumferentially around a cylindrical chamber to increase the plasma density. Microwave power is injected into the chamber perpendicular to the longitudinal axis of the chamber.
U.S. Pat. No. 5,241,244 to Cirri describes a plasma generating device having a confinement field and an electromagnetic field. Permanent magnets are disposed at one end of the plasma chamber, and a coil is disposed circumferentially around the chamber.
U.S. Pat. No. 4,891,095 to Ishida et al. describes a plasma generating device in which a mirror field has its field axis parallel to a specimen surface. The mirror field is supplied by a pair of electromagnets disposed on diametrically opposite sides of the reaction chamber.
U.S. Pat. No. 5,032,202 to Tsai et al. describes a plasma generating device which uses a solenoid magnet disposed on the axis of the chamber, and a plurality of permanent magnet columns disposed circumferentially to radially confine the plasma. Upper and lower ring magnets help confine the plasma in the axial direction.
U.S. Pat. No. 4,778,561 to Ghanbari describes an ECR plasma source having an electromagnet primary field source and a second field source made of circumferentially disposed permanent magnets which are axially spaced from the primary source.
In general, ECR zones in ECR ion sources are limited to regions where the magnetic field meets the ECR resonant condition, given by the following expression:
.omega..sub.ECR =Be/m=.omega..sub.n
where .omega..sub.ECR is the electron-cyclotron resonant angular frequency, .omega..sub.n is the resonant frequency of the microwave power source, e is the charge, and m is the mass of the electron.
Whenever the microwave frequency is tuned to the electron-cyclotron frequency, electrons can be resonantly excited and thereby given sufficient energy to cause ionization within an evacuated volume. At low collision frequencies (low ambient pressures), some of the electrons are coherently excited and given very high energies which are capable of removing tightly bound electrons and, therefore, are responsible for producing multiply ionized atoms.
In conventional ECR ion sources, tuning to the resonant condition is accomplished by adjusting the magnitude of the mirror coils, located at the ends of the ionization chamber. The geometry and design principles used in conventional ECR sources are illustrated in FIG. 1. FIG. 1 is from a publication entitled "ECR Sources For The Production Of Highly Charged Ions (Invited)" by C. M. Lyneis et al., Rev. Sci. Instrum. 61 (1) (Jan. 1990). In FIG. 1, the axial magnetic field corresponding to a 250 amp current in the coils is superimposed on the drawing.
This type of ECR ion source design has several inherent limitations on performance in terms of the degree of ionization (intensity) and high charge state capabilities. For one limitation, the ECR zones are limited to localized regions of the source ionization volume where the magnetic field flux density is correct for resonant excitation. These zones are small with respect to the total ionization volume and are fluted ellipsoidal surfaces which surround the axis of symmetry where the ions are extracted from the source. The ECR surfaces shift in position whenever the magnetic field is adjusted.
For another limitation, because of the small physical size of the ECR zones in relation to the total ionization volume and the fact that only electrons within this zone are excited, electrons within most of the plasma volume are not excited. As a consequence, the "hot" zones are very small compared to the "cold" zones within the chamber. The "cold" zones are filled with lower-charge-state and neutral atoms. Multiply charged ions, created by "hot" electrons, undergo resonant or perhaps non-resonant charge exchange during collisions with like neutral or lower-charge-state species more frequently in the "cold" zones because of the higher population of these species in these zones. This process results in much lower average charge states and degree of ionization than could be achieved if the ECR zones filled most of the cavity (The resonant charge exchange process has the characteristic feature that the cross section is maximum at zero center of mass energies). Non-resonant charge exchange processes (collisions between unlike species) also occur which further reduce the average charge state and degree of ionization within the source.
The effect of reducing the probability of charge exchange as a consequence of increasing the ECR zone would be to increase the residence time of an ion species in a particular charge state, thereby increasing the probability of further ionization of the species. Thus, incorporation of a large ECR zone would be desirable for high-charge-state sources, as well as high-intensity, low-charge-state ion source applications. Since the "hot" zones lie off the optical axis or axis of symmetry of the source, ions must always pass through extended "cold" zones of the source prior to extraction. Furthermore, an extended "cold" zone occurs at the extraction end of the source. The consequences of this design limit the capabilities of the ECR ion source in terms of intensity and charge state distribution.
Because high ECR ion sources work at low pressures (1.times.10.sup.-6 Torr, typically), the amount of microwave power that can be coupled into the plasma is limited. This results in a saturation effect which is often observed experimentally. The amount of power could be increased by increasing the ECR hot zone in relation to the total ionization volume of the source. The result would be to increase the hot zones, thereby increasing the degree of ionization and, as a consequence, reducing the resonant charge exchange processes.
As a further limitation of the conventional sources, those of the type depicted in FIG. 1 are erroneously said to perform better in terms of charge-state distribution whenever the RF excitation frequency is increased (R. Geller, IEEE Trans. Nucl. Sci. CH2669-0, 1088 (1989)). This finding is attributable to the effect produced by increasing the magnetic field strength of the mirror coils which, increases the confinement times of particles in the plasma. In order to maintain a constant position of the ECR zone, higher frequency power sources are required; this observation is, therefore, not a fundamental attribute of the RF frequency. The advantages of using higher frequency microwave power sources lie in the higher plasma densities that can be achieved without cutoff problems. The consequence of increasing the magnetic field is to increase the containment time of a particle in a particular charge state, thereby increasing the charge state distribution from the source. The penalties paid for increasing the magnetic field are higher power requirements and, consequently, power costs for operating the source. Typical sources require up to 5,000 Gauss fields and use 14 GHz microwave power sources.
Emittance brightness degradation is a further limitation of the conventional ECR devices. The emittances of conventional ECR ion sources are, generally, higher than other positive ion sources due to two effects; both effects are caused by the very strong mirror fields used to confine the plasma in the axial direction. Ions are extracted across strongly graded and radially directed magnetic fields, which results in emittance degradation due to conservation of angular momentum and due to the non-linear coupling of longitudinal momentum to the transverse directions of motion. These effects increase the phase space (emittance) .epsilon. approximately proportional to .DELTA..epsilon..alpha.Br.sup.2 Thus, lower emittances can be effected by reducing the magnetic field strength in the main volume of the source and in the extraction regions of the source. Emittance degeneration can be reduced by using a relatively low RF frequency power source for plasma ignition and maintenance while providing magnetic shields to shield the ions during extraction from the effects of the strong mirror fields in the extraction region of the source.